A technology, in which an exhaust performance and fuel economy can be enhanced or improved by performing EGR (exhaust-gas recirculation) for introducing a part of exhaust gas into an intake system depending on an operating condition in an internal combustion engine equipped with an EGR passage connected to an exhaust passage and an intake passage and an EGR control valve disposed in the EGR passage, is generally known.
On the assumption that the differential pressure (the pressure difference) between an intake pressure in the intake passage at a position of the intake passage at which the EGR passage is connected and an exhaust pressure in the exhaust passage at a position of the exhaust passage at which the EGR passage is connected is kept constant, the amount of EGR, introduced into the intake passage, is determined depending on the opening degree of the EGR valve disposed in the EGR passage. In such internal combustion engines configured to perform EGR, when being used for a long period of time, an EGR rate, determined depending on the opening of the EGR valve, tends to vary from a desired value owing to a change in pressure loss of an intake and exhaust system. Hence, there is a possibility that the amount of EGR, actually recirculated, deviates from a target value.
Therefore, Patent document 1 teaches a technology that makes a diagnosis on the presence or absence of a change in pressure loss of an intake and exhaust system based on a variation of intake temperature before and after the amount of EGR, which is introduced into an intake passage, is varied in a given transient state.
However, in making a diagnosis on the presence or absence of a change in pressure loss of an intake and exhaust system based on a variation of intake temperature, suppose that the temperature of EGR gas, introduced into the intake passage, is low. In such a case, a variation of temperature of intake air, occurring due to the EGR gas sent back, becomes smaller. For that reason, in the case that an EGR cooler is installed in the EGR passage such that the EGR gas, introduced into the intake passage, is cooled, there is a possibility that it is difficult to make a diagnosis on the presence or absence of pressure loss in an intake and exhaust system based on a variation of intake temperature.